1. Field of the Invention
This invention relates generally to safety pressure relief devices and more particularly, but not by way of limitation, to a safety pressure relief device designed for positive rupture in one direction at a pre-determined pressure differential to protect a pressure vessel. The invention also relates to methods for making such safety pressure relief devices.
2. Description of the Prior Art
A variety of safety pressure relief devices of the rupture disk type have been developed. Generally, these devices include a rupture disk supported between a pair of complementary supporting members or flanges which are in turn connected to a relief connection in a vessel or system containing fluid pressure.
Safety pressure relief devices of the rupture disk type are often used in environments having elevated temperatures, alternating pressure and vacuum and corrosive conditions. When such disks are exposed to alternating internal and external pressures the central portion of the disk fluctuates responsive to the pressure diferentials thus stressing the disk and ultimately leading to disk rupture at a pressure differential other than that for which the disk was designed. As used herein, the term "positive pressure" refers to a condition in which pressure in the vessel is greater than pressure on the other side of the safety pressure relief device while the term "reverse pressure" refers to a condition in which the vessel pressure is less than the pressure on the other side of the presure relief device.
In order to stablize the rupture disk against fluctuating in response to alternating internal and external pressure differentials, a vacuum support is clamped between the complementary flanges which contain the rupture disk with the vacuum support being positioned internally of the rupture disk. In some constructions, a sealing member is interposed between the vacuum support and the rupture disk. Thus, the vacuum support prevents fluctuation of the rupture disk or seal when a vacuum occurs in the pressure vessel in which the safety pressure relief device is installed. Examples of such supports are shown in U.S. Pats. No. 2,523,068 to Simpson et al. and U.S. Pat. No. 2,953,279 to Coffman.
A problem occurs with such prior art vacuum supports. In many applications, the vacuum support must be thick enough to support the rupture disk or seal in the presence of a high reverse pressure. At the same time, the vacuum support must also be able to open completely upon the occurrence of the positive pressure at which the rupture disk is designed to open. When the rupture disk is designed to open at a relatively low positive internal pressure, the vacuum support may be so thick that full opening of the assembly at the pre-determined positive pressure is prevented thus restricting flow.
Prior art vacuum supports typically include a plurality of petals defined by radial cuts in the vacuum support. Lugs are welded onto the vacuum support adjacent the cuts in order to maintain the structural integrity of the vacuum support in the presence of reverse pressure while a sufficient pre-determined positive pressure opens the petals upon bursting of the rupture disk. In the past, it has been necessary to design vacuum supports of varying thicknesses dependent upon the level of reverse pressure the vacuum support must withstand. Moreover, some design parameters are not achieveable using prior art vacuum supports. In other words, when it is desired to have the vacuum support withstand a very high reverse pressure while opening in the presence of a very low positive pressure, the vacuum support may be so thick that rupture of the disk at the positive pressure may not occur.
In addition to the foregoing problems, the lugs which are spot welded onto the prior art vacuum supports constitute surface irregularities upon which process in the vessel may accumulate. When such process is corrosive, especially in the presence of high temperatures, the effective life of the vacuum support is reduced.
It is important that when the internal pressure in the vessel reaches the pre-determined positive pressure at which the rupture disk opens, the vacuum support opens completely and fully while at the same time retaining parts so that vacuum support fragments are not sent downstream of the safety pressure relief device.
By the present invention, a safety pressure relief device of the rupture disk type is provided which includes a vacuum support which is simply and easily constructed, which opens completely and positively at low rupture disk pressures while supporting the rupture disk in the presence of a high reverse pressure, which is useable with rupture disks having a wide range of reverse pressure and rupture pressure ratings, and which opens positively and completely without fragmenting when internal vessel pressure exceeds rupture disk opening pressure.